This invention relates to high power (nonlinear) testing of microwave transistors (DUT). When the transistor is driven in its nonlinear operation regime, whereby they create also harmonic signal components and the internal output impedance of power transistors is very low. An impedance tuner used to match the transistor must also match such impedance. Passive impedance tuners can reach maximum reflection factors Γ.tuner of the order of 0.95, corresponding to impedances of 2.4 Ohm. The insertion loss between DUT and tuner reduces the available tuning range at the DUT reference plane and thus the capacity of the passive tuner to match the transistor. The only remedy to this limitation is using active systems, i.e. test systems whereby a signal coherent with the signal injected into the transistor, is injected independently into the DUT output terminal and creates a virtual load. This additional signal can be the only one injected, in which case we speak of “active” load pull, or is can be superimposed to signal reflected by a passive tuner, in which case we speak of “hybrid” load pull; obviously if only a mechanical tuner is present, we speak of “passive” load pull. In both active and hybrid injection cases the objective is to reach and match the internal impedance of the transistor; in general terms a standard requirement is a dynamic tuning range reaching a reflection factor |Γ|=1 (corresponding to the real part of an internal impedance of 0 Ohm). The objective of this invention is a hybrid (active and passive) tuner apparatus, combining a forward signal injection mechanism within a passive tuner architecture allowing |Γ|=1 at two harmonic frequencies (see ref. 6 and 9).